Who do you love?
by you.broke.a.promise
Summary: Derek's in love, but the real mystery is Casey's strange behavior.


Disclaimer: I do not own Life With Derek! However, if anyone wants to send Michael Seater my way, I'd be eternally grateful :)

Rated M for sexual situations, although there is no actual lemon.

* * *

She was bent over her notebook when he approached, so caught up in her math homework that she didn't even hear him stop at her desk. The room had long since been empty and she had started spending some her lunches in the desolate mathematics room after she'd broken up with Max. _Some_ had turned into _all_ when Derek had started dating Sally.

He watched her gaze flick back and forth from her notebook to her text book for a few moments, erasing and re-writing, erasing and re-writing. She jumped when he said her name.

"Jesus Christ, Derek! Yodel or something when you approach!"

He let out a low chuckle and plopped into the chair at the desk next to hers, "But then I wouldn't get to see you crawl out of your skin."

Casey frowned, turning her attention back to her text book, "What do you want?"

Derek sighed, propped his feet up on the desk and leaned back, "Can't I just come to say hello?"

The brunette turned to him, her expression incredulous, "No? You're incapable of civil conversation. Something's up. You want something. What is it?" Out of the corner of her eye, she watched him shrug out of his jacket.

"Actually, Space Case, I don't. I just needed to get out of the cafeteria."

Casey's pencil paused in its decent across another equation, "Mr. Popular finally get tired of all the attention?"

"Maybe I filled Sam's lunch bag with aerosol cheese and didn't want to be there when he found out."

"Liar, you wouldn't do that to Sam. That was the worst excuse I've ever heard you use, and I've heard you use some pretty stupid ones." Her eyes were still on her homework.

Derek looked at the wall, "I'm avoiding Sally."

"Oh, Derek! That's mean." Yet she didn't even look up to scold him. He turned to look at her and she sighed, putting down her pencil and turning her head towards the window, "Why are you avoiding her? Had enough?"

"She's like in love with me, Case. And I'm just… not."

"That's unsurprising," She snorted and went back to her scribbling. However, Derek's lack of sarcastic retort even after a few minutes had her turning towards him. There was a new layer of concern to her voice when she spoke, "What's the matter, D?"

"Nothing," He replied too quickly, "Don't worry about it, Ms. Busy Body."

Her nose wrinkled, "Busy body? That insult was even lamer than the last. So lame, I'm completely sure there's something wrong now." She stared at him, waiting.

"I'm just in love with somebody else."

"Yourself?" She asked sarcastically. There was no real question there.

Derek scoffed, "No."

Casey blinked. "Emily?"

"Ugh, no." He watched as Casey's eyebrows came together in thought, "Jeez Case, don't hurt yourself."

She frowned at him and closed her text book, "Kendra?"

"Absolutely not," His face scrunched up in distaste, "You thought I'd be in love with Emily or Kendra but not Sally?"

"Not really," she admitted, "I'm just running low on ideas."

Casey McDonald, running low on ideas?" Derek let out a low whistle, "Damn. I must be really good or you must be really dumb." Her eyes narrowed as she realized either way, he won. He grinned back at her. "We both know you're not stupid."

"Oh, so you must be really good then," She threw back sarcastically. "Yes, you're fantastic. I almost feel bad for this mystery girl, having to put up with your…" She made air quotes, "affections."

A look of hurt flashed through Derek's eyes before he pushed it away and stood. He'd moved so abruptly that she flinched away, expecting some sort of aggressive onslaught.

"Think I'm going to hurt you, Case?" He asked, his voice low.

"Well, you do have a tendency to poke, prod and pinch."

She heard the waver in his voice, "Touché."

Suddenly feeling self conscious, Casey slipped away from her desk and moved up to the chalk board at the front of the room, taking her notebook with her. She opened it quietly and looked at it for a moment before putting it down on the teacher's desk. Her hand shook as she picked up the chalk. She only got half of the problem up when his lips touched her ear, "You know better than to turn your back on the enemy."

She jumped and turned, back against the chalk board, "What are you playing at here, Derek?"

He sighed and leaned back on the teacher's desk, running a hand through his hair. Frustration was evident on his features. "Nothing."

He turned away from the desk and moved to retrieve his brown leather jacket from where he'd been sitting.

"Wait." When he turned, Casey was already standing behind him. He absently decided that it was her dancer's grace that made her foot falls so quiet on the hard linoleum floor. She was twisting her hands, her eyes downcast as she spoke, "You never told me who you were in love with."

After a moment she looked up at him. He stared at her. Their eyes locked and with reflexes only Derek Venturi could have, he leaned forward, grasped her face and slanted his lips across hers. Her expression was a mask of shock for the first few milliseconds before she started to respond, and then it became all raw need and built up sexual tension.

It was the bell signaling the end of the period that broke them apart. Casey was breathing raggedly, her cheeks flushed.

"You'll figure it out someday, Spacey." And he left the room with a smirk.

She somehow managed to avoid him for the rest of the day. Unfortunately, he had hockey practice after school, so cornering her would have to wait a bit longer than he wanted. She wasn't home when Sam dropped him off, the only car in the driveway was Nora's.

He pushed the front door open and dropped his sports bag on the floor. After a moment of thought, he shut his eyes, simultaneously calling out "I'm home!" and kicking the front door shut with his heel. There was no response to his call, and he flew up the stairs. "Ed? Marti?"

Silence.

The house to himself? It was a rare moment, and he wanted to take advantage of it. Any other day he'd have gone straight for Babe Raider and his chair, but today he was disgustingly sweaty and although he'd defiled his chair in many, _many_ ways, he refused to do something to it that would make it smell. All his thoughts of Casey were momentarily forgotten.

He was soaping up his chest when the door opened. His head snapped up and through the top half of the shower's glass door, his eyes met Casey's. They stared at each other for a minute and then it dawned on Derek—

"Holy shit, Case!"

He only just seemed to remember he was naked. Casey however, seemed unfazed. He slammed his hand over the shower knob, effectively cutting off the water. In quick, panicked movements, he opened the door and groped for his towel, only suddenly realizing he'd have to completely expose himself to her to get it. She had a smirk on her lips when she glided forward and yanked the towel off the hook to hand it to him. His eyes crossed for a second and he snatched the towel out of her hand and wrapped it around his hips before stepping out of the confines of the shower.

She started to snigger.

"Oh yeah, you think you're so hilarious," He grumbled, shutting the shower door behind him. "Was there a reason you busted in on my naked time?"

_You're in love with me._

She grinned at him, "I just wanted to see you when you were vulnerable."

His face got hot. "More like you wanted to see me naked."

"I will admit your abs are one of the only good things about you." Her leer was unmistakable.

He gaped at her. Who was this girl and what had she done with Casey McDonald? It had to be a prank. A sick, cruel, evil prank. Well, he could play her game. His mouth snapped shut so fast he was sure she heard his teeth clink together. A lewd sneer slid over his mouth, "Oh yeah? You wanna see them up close?"

Casey batted her eyelashes innocently.

"You got it, Princess." He sauntered over to her, water drops still sliding down his torso and grabbed her hand. She watched with what seemed to be curiosity as he ran the captured appendage over his abdomen.

And then she lunged.

His breath caught in his throat as they tumbled to the cold, white and green tiled floor. Her mouth was hot on his and he was suddenly hyper aware of the way her knees squeezed into his hips. Her hands were in his hair, which was still sopping wet. She bit his lip.

"Who are you?" He asked suspiciously. He was stunned. If this was a prank, she really had him going.

"What do you mean, D?" Her voice was breathy.

"Y-you! But! You jumped me! You actually jumped me!" His eyes were drawn to her white tank top, which was now wet from being pressed up against his chest. Her bra was light blue lace.

"Yes I did," She agreed.

"Why!" He was still convinced it wasn't real. "And where is everyone?"

"George's car battery died… Mom's got something in the back of her car so she took the kids in the Prince to go jump it. And why? Well, that's simple. Me."

"You?"

"Me," she repeated.

"I don't get it."

She laughed, and it jolted him to realize that he enjoyed the sound. "Should that come as a surprise to me? I can practically hear the water sloshing around in your skull. There might be a few ice cubes in there as well. But that's only if you're lucky." She laughed again at his expression. "Not Emily, not Sally, not Kendra. Me."

"You," he agreed.

She tugged on a lock of his hair, "Mhm."

He blinked at her. "Well who knew," A grin widened his mouth, "Spacey figured it out."

"Hell yes she did," And her mouth descended on him again. This time it was a bit farther south. He clenched his eyes shut as her tongue slid over his previously mentioned abs.

"What are you doing?" His mind was already going places he knew it shouldn't. How was he supposed to control himself with her all needy and breathing on top of him? And he was hardly clothed… _Aaaagh. Old women. Dogs. Edwin. _

"What does it look like I'm doing?" She stopped her ministrations and peered into his face, "Oh wait, you can't see. Your eyes are closed."

He could practically feel the smugness radiating off of her. He waited a second and then opened his eyes. _Yep, smug_.

"Are you sure?" It was all he could come up with.

"If I wasn't sure, would I do this?" And her hand breached the edge of the towel and crawled up his thigh.

"Probably not," His voice was strained- he had imagined this for so long and now that it was actually about to happen, he was sort of scared. Not that Derek Venturi would admit to being scared. _Ever. Ever. Everrrr._ She was teasing him, rubbing circles into his upper thigh. So close, but so far.

Her tongue slid across his collarbone and he gave up fighting. His hands grasped the hem of her shirt and yanked upwards, revealing the damnable blue lace bra. He glared at it for a moment before he was successfully distracted. Her hand had finally found its destination and he bit his lip hard enough to taste blood.

He grabbed her face and mashed their lips together, not caring whether or not anything else in the world existed. Who cared if she was his step sister? They weren't related by blood… they could have babies… and the babies wouldn't be deformed… they'd be cute with her eyes and his smile and her legs and his hair.

_Yeah._

There was no blood between them. Nope. None. Zero. Nada. So this wasn't wrong. Or illegal. He'd even had Edwin look it up for him once, joking that he'd seen a movie in which the step siblings got a little freaky and wanted to know if he should sue for emotional trauma. He could recall Edwin's smirk and raised eyebrow. He was pretty sure Ed knew how he felt about Casey anyway… he'd accidentally played footsie with his brother at dinner a few times instead of her. There were smirks and raised eyebrows those times too.

He reached up behind her and unclasped her bra, and then yanked it off completely. He grinned at the sight of her looking slightly startled at his sudden aggressiveness, but she just smiled at him and sped up her gesticulations.

He attacked her chest with his mouth and she bit down on her tongue to keep high pitched squeal from escaping her. She pushed his head back and kissed him forcefully, her hair forming a curtain around his face. Then suddenly it was gone. He groaned at the loss of her warmth when she removed herself from his lap.

"Going somewhere?"

"No, but your towel is."

Cold air swooped in on him but his little Derek managed to remain ridiculously happy. He sat up and yanked her down onto his lap, groping her in the process. She yelped and bit down on his shoulder, her hand moving in-between them to grasp what lie between his legs.

* * *

When the family returned home later, Casey was sitting in Derek's recliner.

"Uh oh," Edwin grinned as he walked through the door, "Derek must not be home yet. You better get up before he gets here because the second he sees you in his chair he's gonna—"

Even Marti's mouth fell open when the family spied Derek sprawled leisurely across the couch cushions, his head resting on the arm of the couch near the chair. "He's gonna what, Ed?" Derek asked without any real interest, not looking away from the television screen.

"Smerek! Are you sick!" Marti bounded over to the couch to feel his forehead.

"Definitely not, Smarti. Just feeling generous," He turned his head and grinned at the mix of Venturis and McDonalds standing in the doorway. Casey glanced over at them and flashed them a grin that mirrored Derek's. Edwin figured he could get every other member of the family to agree that it was the scariest thing they'd ever seen. Lizzie was the first to recover, and she shut the door behind them with a loud thud.

"U-uh," George said intelligently.

Marti, recovering from shock, scrambled away from Derek and latched onto George's leg. Neither Derek nor Casey seemed to notice that anything was out of the ordinary and both turned their attention back to the television.

"Daddy... I'm scared," Marti whispered.

"Let's just… act normal," Nora managed, giving Edwin a little push away from the front door. "You and Lizzie are supposed wash dishes tonight, right?"

"Uhm, sure," Edwin volunteered, pulling the youngest McDonald behind him. They disappeared into the kitchen.

"Marti, would you like to play Go Fish?" Nora asked, still looking anywhere but the living room.

"Okay, Nora," Marti agreed, pulling on George's pant leg. "Dad plays too."

"Sure, Marti," George had finally found his voice. As Marti disappeared to find the cards, George leaned towards Nora to whisper in her ear, "Should we be worried?"

Nora's gut told her yes. The satisfied looks on the faces of Derek and Casey told her yes. The way Casey's hand dangled dangerously close to Derek's hair told her yes. _Yes yes yes yes_. She looked at George, "No, everything's going to be fine."

* * *

Hehe. :)


End file.
